Display device

ABSTRACT

Provided is a display device including: a capacitor having a first electrode, a first insulating film over the first electrode, and a second electrode over the first insulating film; and a first transistor over the capacitor. The first transistor includes the second electrode, a second insulating film over the second electrode, an oxide semiconductor film over the second insulating film, and a first source electrode and a first drain electrode over the oxide semiconductor film. The first source electrode and the first drain electrode are electrically connected to the oxide semiconductor film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority from theprior Japanese Patent Application No. 2016-074019, filed on Apr. 1,2016, the entire contents of which are incorporated herein by reference.

Field An embodiment of the present invention relates to a displaydevice, for example, a display device such as an organic EL displaydevice and a manufacturing method thereof. Background

As a typical example exhibiting semiconductor properties, Group 14elements such as silicon and germanium are represented. Particularly,silicon has been utilized in almost all of the semiconductor devices andrecognized as a material supporting the basis of the electronicsindustry because of its wide availability, ease of processing, excellentsemiconductor properties, and ease of controlling properties.

Recent finding of semiconductor properties in oxides, in particularoxides of Group 13 elements such as indium and gallium, has motivatedintensive research and development. As a typical example of oxidesexhibiting semiconductor properties (hereinafter, referred to as anoxide semiconductor), indium-gallium oxide (IGO), indium-gallium-zincoxide (IGZO), and the like have been known. Intensive research in recentyears has realized commercialization of display devices havingtransistors including these oxide semiconductors as a semiconductorelement. Additionally, as exemplarily disclosed in Japanese patentapplication publication No. 2015-225104, international publication No.2015-031037, and United States patent application publication2010/0182223, a semiconductor device in which both a transistor having asilicon-including semiconductor (hereinafter, referred to as a siliconsemiconductor) and a transistor having an oxide semiconductor areincorporated has been developed.

SUMMARY

An embodiment of the present invention is a display device including: acapacitor having a first electrode, a first insulating film over thefirst electrode, and a second electrode over the first insulating film;and a first transistor over the capacitor. The first transistor includesthe second electrode, a second insulating film over the secondelectrode, an oxide semiconductor film over the second insulating film,and a first source electrode and a first drain electrode over the oxidesemiconductor film. The first source electrode and the first drainelectrode are electrically connected to the oxide semiconductor film.

An embodiment of the present invention is a display device including: acapacitor having a first electrode, a first insulating film over thefirst electrode, and a second electrode over the first insulating film;and a first transistor over the capacitor. The first transistor includesthe second electrode, a second insulating film over the secondelectrode, an oxide semiconductor film over the second insulating film,a third insulating film over the oxide semiconductor film, a thirdelectrode over the third insulating film, and a first source electrodeand a first drain electrode over the third electrode.

The first source electrode and the first drain electrode areelectrically connected to the oxide semiconductor film.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a display device of an embodiment of thepresent invention;

FIG. 2 is a schematic top view of a display device of an embodiment ofthe present invention; FIG. 3 is an equivalent circuit of a pixel of adisplay device of an embodiment of the present invention;

FIG. 4 is a schematic top view of a pixel of a display device of anembodiment of the present invention;

FIG. 5 is a schematic cross-sectional view of a pixel of a displaydevice of an embodiment of the present invention;

FIG. 6A to FIG. 6C are schematic cross-sectional views showing amanufacturing method of a display device of an embodiment of the presentinvention;

FIG. 7A to FIG. 7C are schematic cross-sectional views showing amanufacturing method of a display device of an embodiment of the presentinvention;

FIG. 8A and FIG. 8B are schematic cross-sectional views showing amanufacturing method of a display device of an embodiment of the presentinvention;

FIG. 9A and FIG. 9B are schematic cross-sectional views showing amanufacturing method of a display device of an embodiment of the presentinvention;

FIG. 10 is a schematic cross-sectional view showing a manufacturingmethod of a display device of an embodiment of the present invention;

FIG. 11 is a schematic cross-sectional view of a display device of anembodiment of the present invention;

FIG. 12 is an equivalent circuit of a pixel of a display device of anembodiment of the present invention; FIG. 13 is a schematic top view ofa pixel of a display device of an embodiment of the present invention;

FIG. 14 is a schematic top view of a pixel of a display device of anembodiment of the present invention;

FIG. 15 is a schematic cross-sectional view of a pixel of a displaydevice of an embodiment of the present invention;

FIG. 16 is a schematic top view of a pixel of a display device of anembodiment of the present invention; and

FIG. 17 is a schematic cross-sectional view of a pixel of a displaydevice of an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the present invention are explained withreference to the drawings. The invention can be implemented in a varietyof different modes within its concept and should not be interpreted onlywithin the disclosure of the embodiments exemplified below.

The drawings may be illustrated so that the width, thickness, shape, andthe like are illustrated more schematically compared with those of theactual modes in order to provide a clearer explanation. However, theyare only an example, and do not limit the interpretation of theinvention. In the specification and the drawings, the same referencenumber is provided to an element that is the same as that which appearsin preceding drawings, and a detailed explanation may be omitted asappropriate.

In the present invention, when a plurality of films is formed byprocessing one film, the plurality of films may have functions or rulesdifferent from each other. However, the plurality of films originatesfrom a film which is formed as the same layer in the same process.Therefore, the plurality of films is defined as films existing in thesame layer. In the specification and the scope of the claims, unlessspecifically stated, when a state is expressed where a structure isarranged “over” another structure, such an expression includes both acase where the substrate is arranged immediately above the “otherstructure” so as to be in contact with the “other structure” and a casewhere the structure is arranged over the “other structure” with anadditional structure therebetween.

First Embodiment

In the present embodiment, structures of a semiconductor deviceaccording to an embodiment of the present invention and a display deviceincluding the semiconductor device are explained by using FIG. 1 to FIG.5.

1. Outline Structure

A perspective view schematically showing a display device 100 accordingto the present invention is illustrated in FIG. 1. The display device100 has a display region 106 including a plurality of pixels 104arranged in a row direction and column direction and gate-side drivercircuits 108 over one surface (top surface) of a substrate 102. Thedisplay region 106 and the gate-side driver circuits 108 are providedbetween the substrate 102 and an opposing substrate 116.

Display elements such as a light-emitting element and a liquid crystalelement giving colors different from one another can be disposed in theplurality of pixels 104, by which full-color display can be conducted.For example, display elements providing red, green, and blue colors maybe arranged in three pixels 104, respectively. Alternatively, displayelements exhibiting white color may be used in all pixels 104, andfull-color display may be performed by using a color filter to extractred, green, or blue color from the respective pixels 104. The colorsfinally extracted are not limited to a combination of red, green, andblue colors. For instance, four kinds of colors of red, green, blue, andwhite may be respectively extracted from four pixels 104. Thearrangement of the pixels 104 is also not limited, and a stripearrangement, a delta arrangement, a Pentile arrangement, and the likecan be employed.

As shown in FIG. 2, the display device 100 further possesses asource-side driver circuit 110. A variety of wirings exemplified byfirst scanning lines 120, second scanning lines 126, and image-signallines 122 extends in a direction to the display region 106 from thegate-side driver circuits 108 and the source-side driver circuit 110 andis connected to the respective pixels 104. Wirings 114 extend to a sidesurface of the substrate 102 (short side of the substrate 102) from thesource-side driver circuit 110 and are exposed at an end portion of thesubstrate 102 to form terminals 110. The terminals 110 are connected toa connector (not shown) such as a flexible printed circuit (FPC). Imagesignals supplied from an external circuit (not shown) are provided tothe pixels 104 through the gate-side driver circuits 108 and thesource-side driver circuit 110, and the display elements of the pixels104 are controlled, by which images are displayed on the display region106. In the present embodiment, two gate-side driver circuits 108 arearranged so as to sandwich the display region 106. However, just asingle gate-side driver circuit 108 may be employed. Furthermore, thegate-side driver circuits 108 and the source-side driver circuit 110 maynot be directly formed over the substrate 102, and a driver circuitfabricated over a different substrate may be mounted over the substrate102 or the connector.

2. Pixel Circuit

FIG. 3 shows an equivalent circuit of the pixel 104. The pixel 104 has aplurality of wirings and a plurality of semiconductor elements such as atransistor and a capacitor. Specifically, the first scanning line 120,the second scanning line 126, a third scanning line 128, theimage-signal line 122, a reset power-source line 130, and acurrent-supplying line 124 are provided in the pixel 104. The firstscanning line 120, the second scanning line 126, and the third scanningline 128 extend in the row direction from the gate-side driver circuit108 through the plurality of pixels 104. On the other hand, the imagesignal line 122 and the reset power-source line 130 intersect with thefirst scanning line 120, the second scanning line 126, and the thirdscanning line 128 substantially perpendicularly and extend in the columndirection through the plurality of pixels 104.

The pixel 104 possesses, as a semiconductor device, a first transistor140, a second transistor 142, a third transistor 144, a fourthtransistor 146, a capacitor (storage capacitor) 148, and the displayelement 150. Note that the equivalent circuit shown in FIG. 3 is just anexample: the number of the transistors is not limited to four and may betwo, three, five, or more, for example. Moreover, the number of thecapacitors is not limited to one, and a plurality of capacitors may beincluded. The aforementioned combination of the wirings is also anexample: another wiring may be disposed, and a part of theaforementioned wirings may be shared by the plurality of pixels 104.

The display element 150 is selected from a liquid crystal element and alight-emitting element. As a light-emitting element, a self-emissiontype element such as an organic electroluminescence (EL) element isrepresented. The display element 150 includes a pixel electrode, anopposing electrode, and a liquid crystal layer or an EL layer sandwichedtherebetween.

The second scanning line 126 is connected to a gate electrode of thethird transistor 144, and a control signal BG is input to the gateelectrode of the third transistor 144. A source electrode of the thirdtransistor 144 is connected to the current-supplying line 124 andapplied with a high potential PVDD. A drain electrode of the thirdtransistor 144 is connected to a drain electrode (first drain electrode)of the first transistor 140.

A gate electrode of the second transistor 142 is connected to the firstscanning line 120 and applied with a control signal SG. A sourceelectrode (second source electrode) of the second transistor 142 isconnected to the image-signal line 122 and is input with an image signalVsig or an initializing signal Vini. A drain electrode (second drainelectrode) of the second transistor 142 is connected to a gate electrodeof the first transistor 140 and one electrode (second electrode) of thecapacitor 148.

A source electrode (first source electrode) of the first transistor 140is connected the other electrode (first electrode) of the capacitor 148and the pixel electrode (anode) of the display element 150.

A gate electrode of the fourth transistor 146 is connected to the thirdscanning line 128 and input with a control signal RG. A source electrodeof the fourth transistor 146 is connected to the reset power-source line130 and is applied with a reset potential Vrst. A drain electrode of thefourth transistor 146 is electrically connected to the other electrode(first electrode) of the capacitor 148 and the pixel electrode of thedisplay element 150.

The opposing electrode (cathode) of the display element 150 is appliedwith a low potential PVSS.

During operation, the control signal RG is input to the gate electrodeof the fourth transistor 146 turning the fourth transistor 146 to anon-state. With this operation, the reset potential Vrst is provided tothe first electrode of the capacitor 148, the pixel electrode, and thesource electrode of the first transistor 140 from the reset power-sourceline 130, by which a potential of the source electrode of the firsttransistor 140 is reset.

After that, the gate electrode of the second transistor 142 is appliedwith the control signal SG turning the second transistor 142 to anon-state.

At this time, the image-signal line 122 is applied with the initializingsignal Vini to allow the gate electrode of the first transistor 140 tohave a potential corresponding to the initializing signal Vini, by whichinitialization is performed.

Next, the fourth transistor 146 is turned off, the gate electrode of thesecond transistor 142 is input with the control signal SG to turn on thesecond transistor 142, and the gate electrode of the third transistor144 is input with the control signal BG to turn off the third transistor144. With this process, an offset-canceling operation of a threshold ofthe first transistor 140 is performed. At this time, the image-signalline 122 is applied with the initializing signal Vini which is input tothe gate electrode of the first transistor 140. A potential of thesource electrode of the first transistor 140 is increased to a potentiallower than the gate potential Vini by a threshold voltage. The potentialdifference at this time depends on the first transistor 140 included ineach pixel 104.

After the offset-canceling operation is completed, the image-signal line122 is input with the image signal Vsig, by which the image signal Vsigis written to the gate electrode of the first transistor 140.

After that, the second transistor 142 is turned off. A current suppliedfrom the current-supplying line 124 flows to the display element 150through the first transistor 140, by which a display operation isstarted, and display is maintained until the third transistor 144 isturned off by the control signal BG.

Note that the source electrode and the drain electrode of eachtransistor may be interchanged depending on a direction of currentflowing through the semiconductor film and a polarity of the transistor.Therefore, the aforementioned relationship between the source electrodeand the drain electrode is simply for the sake of convenience.

3. Pixel Structure

A schematic top view of the pixel 104 is shown in FIG. 4. The displayelement 150 is not illustrated in FIG. 4. The second transistor 142includes a semiconductor film 160, and the image-signal line 122 alsoserves as the source electrode (second source electrode) of the secondtransistor 142. The drain electrode (second drain electrode) 166 of thesecond transistor 142 is connected to the second electrode 172 of thecapacitor 148 through a contact hole (a dotted circle in the drawing.The same is applied hereinafter.), and the second electrode 172 forms apair with the first electrode 170 of the capacitor 148.

The third transistor 144 has a semiconductor film 162, and a part of thesecond scanning line 126 (a portion protruding upward in the drawing)functions as a gate electrode. The semiconductor film 162 iselectrically connected to the current-supplying line 124 and firstsource electrode 182 of the first transistor 140 in contact holes formedthereover.

The fourth transistor 146 has a semiconductor film 164, and a part ofthe third scanning line 128 (a portion protruding upward in the drawing)functions as a gate electrode. The semiconductor film 164 iselectrically connected to the reset power-source line 130 and to thepixel electrode through a connection electrode 190 in contact holesformed thereover. The first drain electrode 184 and the connectionelectrode 190 are integrally formed as a single unit.

The first transistor 140 overlaps with the capacitor 148. Specifically,as shown in FIG. 4, the first transistor 140 includes a semiconductorfilm 180, and the semiconductor film 180 overlaps with the firstelectrode 170 and the second electrode 172. An area of the semiconductorfilm 180 can be smaller than that of the second electrode 172, and thewhole of the semiconductor film 180 may overlap with the secondelectrode 172. The source electrode (first source electrode) 182 and thedrain electrode (first drain electrode) 184 are disposed over thesemiconductor film 180. Therefore, in the first transistor 140, currentflows in a direction parallel to a direction in which the first scanningline 120, the second scanning line 126, and the third scanning line 128extend. The drain electrode (first drain electrode) 184 is electricallyconnected to the first electrode 170 of the capacitor 148 and to thepixel electrode through the connection electrode 190.

A schematic cross-sectional view of the first transistor 140 and thecapacitor 148 overlapping with each other is shown in FIG. 5. FIG. 5 isa cross-sectional view along a chain line C-D in FIG. 4. As shown inFIG. 5, the capacitor 148 is formed over the substrate 102 with anundercoat 192 interposed therebetween. The capacitor 148 includes thefirst electrode 170, the second electrode 172, and a first insulatingfilm 194 sandwiched therebetween. The first insulating film 194functions as a dielectric film of the capacitor 148.

The first transistor 140 is positioned over and overlaps with thecapacitor 148. The first transistor 140 includes the second electrode172 as a gate electrode and possesses a second insulating film 196thereover and the semiconductor film 180 over the second insulating film196. The second insulating film 196 serves as a gate insulating film inthe first transistor 140.

Furthermore, the first transistor 140 has, over the semiconductor film180, the first source electrode 182 and the first drain electrode 184which are electrically connected to the semiconductor film 180. Asdescribed above, the second electrode 172 functions as one of theelectrodes of the capacitor 148 and simultaneously serves as a gateelectrode of the first transistor 140. In other words, the secondelectrode 172 is shared by the capacitor 148 and the first transistor140.

A leveling film 198 is provided over the first transistor 140 andabsorbs projections, depressions, and inclines caused by the capacitor148 and the first transistor 140 to give a flat top surface. There is nolimitation to an element included in the semiconductor films 180, 160,162, and 164 of the first transistor 140, the second transistor 142, thethird transistor 144, and the fourth transistor 146, and silicon,germanium, or an oxide semiconductor is represented. Crystallinity ofthe semiconductor films 180, 160, 162, and 164 is also not limited andmay be single crystalline, polycrystalline, microcrystalline, oramorphous.

In the present embodiment, the first transistor 140 is connected to thedisplay element 150 in series. It is preferred that variation inelectrical characteristics of the first transistor 140, such as athreshold voltage, be small to perform uniform display between thepixels 104. Additionally, a small off-leak current of the firsttransistor 140 is preferred. Therefore, as described below, thesemiconductor film of the first transistor 140 is preferred to includean oxide semiconductor. In this case, the semiconductor films of thesecond transistor 142, the third transistor 144, and the fourthtransistor 146 can contain silicon. Polarity of the second transistor142, the third transistor 144, and the fourth transistor 146 may be an-type or a p-type. When the semiconductor films 160, 162, and 164 ofthe second transistor 142, the third transistor 144, and the fourthtransistor 146 include polycrystalline silicon (polysilicon), a highfield-effect mobility can be obtained.

Structures of the second transistor 142, the third transistor 144, andthe fourth transistor 146 are not particularly limited and may be atop-gate type or a bottom-gate type. In view of the manufacturing methodof the display device 100 described below, a top-gate type is preferred.Moreover, with respect to the positional relationship between thesemiconductor films, the source electrodes, and the drain electrodes,any of a bottom-contact type and a top-contact type may be adopted. Inthe second transistor 142, the third transistor 144, and the fourthtransistor 146, the source electrode and the drain electrode may overlapwith the gate electrode and, alternatively, may not overlap with thegate electrode. The number of the gate electrodes is also notnecessarily single, and the second transistor 142, the third transistor144, and the fourth transistor 146 may have a multi-gate structurehaving two or more gate electrodes.

As described above, variation in electrical characteristics (thresholdvoltage) of the first transistor 140 causes variation in luminancebetween the pixels 104, leading to a reduction in display quality. Thistendency is significantly large particularly in the case where thesemiconductor film 180 includes a polycrystalline silicon (polysilicon)semiconductor. In contrast, variation in characteristics and an off-leakcurrent of a transistor including an oxide semiconductor are relativelysmall compared with those including a polysilicon semiconductor. Hence,the use of an oxide semiconductor in the semiconductor film 180 allowsthe realization of high-quality display.

On the other hand, when the semiconductor film 180 of the firsttransistor 140 includes an oxide semiconductor film, a large currentcannot flow in the first transistor 140 compared with the case where thesemiconductor film 180 is a silicon semiconductor film due to its smallfield-effect mobility. Accordingly, it is difficult to flow a largecurrent in the display element 150 and obtain emission at a highluminance. However, as described above, the first transistor 140 islocated over and overlaps with the capacitor 148 in the semiconductordevice of the present embodiment. The use of such a structure allows anarea occupied by the first transistor 140 to be reduced and a channelwidth to be significantly increased. Hence, it is possible to flow alarge current in the display element 150 and provide an image at a highluminance even in the case where the semiconductor film 180 contains anoxide semiconductor.

Furthermore, as described below, the first insulating film 194 which isthe dielectric film of the capacitor 148 functions as gate insulatingfilms of the second transistor 142, the third transistor 144, and thefourth transistor 146 (see, FIG. 7C). A gate insulating film of atransistor is usually as extremely thin as 50 nm to 100 nm. Thus, it ispossible to increase a capacitance of the capacitor 148. As a result,the potential input to the gate electrode of the transistor 140 and thesecond electrode 172 of the capacitor 148 can be maintained for a longtime. Accordingly, the image signal Vsig can be held for a long time,and a writing frequency of the image signal Vsig can be decreased, whichallows reduction in power consumption.

Second Embodiment

In the present embodiment, a manufacturing method of the display device100 described in the First Embodiment is explained by using FIG. 6A toFIG. 10. Here, an explanation is made by illustratively showing the casewhere a light-emitting element is used as the display element 150, thesemiconductor film 180 contains an oxide semiconductor, and thesemiconductor films 160, 162, and 164 include polysilicon. FIG. 6A toFIG. 10 are cross-sectional views along chain lines A-B, C-D, and E-F.

1. Capacitor First, the undercoat 192 is formed over the substrate 102(FIG. 6A).

The substrate 102 has a function to support each semiconductor elementshown in the First Embodiment. Therefore, a material having heatresistance to a process temperature of each semiconductor element formedthereover and chemical stability to chemicals used in the process may beused. Specifically, the substrate 102 may include glass, quartz,plastics, a metal, ceramics, and the like. When flexibility is providedto the display device 100, a polymer material can be used. For example,a polymer material exemplified by a polyimide, a polyamide, a polyester,and a polycarbonate can be employed. Note that, when a flexible displaydevice 100 is fabricated, the substrate 102 may be called a basematerial or a base film.

The undercoat 192 is a film having a function to prevent impurities suchas an alkaline metal from diffusing to each semiconductor element andthe like from the substrate 102 and can include an inorganic insulatorsuch as silicon nitride, silicon oxide, silicon nitride oxide, andsilicon oxynitride. The undercoat 192 can be formed to have asingle-layer or stacked-layer structure by applying a chemical vapordeposition method (CVD method), a sputtering method, a laminationmethod, and the like. When a CVD method is employed, a tetraalkoxysilaneand the like may be used as a raw material gas. A thickness of theundercoat 192 can be freely selected from a range from 50 nm to 1000 nmand is not necessarily constant over the substrate 102. The undercoat192 may have different thicknesses depending on position. For instance,when the undercoat 192 is configured with a plurality of layers, asilicon nitride-containing layer may be stacked over the substrate 102,and then a silicon oxide-containing layer may be stacked thereover.

When an impurity concentration in the substrate 102 is low, theundercoat 192 may not be provided or may be formed to cover a part ofthe substrate 102. For example, when a polyimide having a lowconcentration of an alkaline metal is employed as the substrate 202, theundercoat 192 may not be provided.

Next, the semiconductor film 160 and the first electrode 170 are formedover the undercoat 192. For example, amorphous silicon (a-Si) with athickness of approximately 50 nm to 100 nm is formed over the undercoat192 with a CVD method and is crystallized by performing a heatingtreatment or irradiation of light such as a laser to transform into apolysilicon film. The crystallization may be carried out in the presenceof a catalyst such as nickel. After that, the polysilicon film isprocessed with etching to form the semiconductor film 160 and the firstelectrode 170. Although not shown, the semiconductor films 162 and 164are formed simultaneously with the semiconductor film 160.

Next, the semiconductor film 160 is masked, and an ion-implantationtreatment or an ion-doping treatment is conducted selectively on thefirst electrode 170. An element such as boron and aluminum imparting ap-type conductivity or an element such as phosphorus and nitrogenimparting an n-type conductivity is represented as an ion. With thisprocess, a conductivity sufficient for the first electrode 170 tofunction as one of the electrodes of the capacitor 148 can be obtained.

Next, the first insulating film 194 is formed over the semiconductorfilm 160 and the first electrode 170 (FIG. 6B). The first insulatingfilm 194 may have a single-layer structure or a stacked-layer structureand include an inorganic insulator usable in the undercoat 192. Similarto the undercoat 192, the first insulating film 194 can be formed byapplying a sputtering method, a CVD method, or the like. The firstinsulating film 194 functions as the dielectric film of the capacitor148 in addition to functioning as the gate insulating films of thesecond transistor 142, the third transistor 144, and the fourthtransistor 146.

Next, a metal film is formed over the first insulating film 194 andsubjected to processing with etching to give the gate electrode 168 ofthe second transistor 142 and the second electrode 172 of the capacitor148 (FIG. 6B). Thus, these electrodes exist in the same layer. In thiscase, the wirings existing in the same layer as these electrodes, e.g.,the second scanning line 126, the third scanning line 128, and the like,are simultaneously formed. It is preferred to adjust the areas of thefirst electrode 170 and the second electrode 172 so that the whole of alower surface of the second electrode 172 overlaps with the firstelectrode 170 or the first electrode 170 is fully covered with thesecond electrode 172 in order to prevent variation in capacitance causedby misalignment.

The metal film can be formed by using a metal such as titanium,aluminum, copper, molybdenum, tungsten, and tantalum or an alloy thereofso as to have a single-layer or stacked layer structure. When thedisplay device 100 of the present invention possesses a large area, theuse of a metal with a high conductivity, such as aluminum and copper, ispreferred in order to avoid signal delay. For example, a structure canbe employed in which aluminum or copper is sandwiched by a metal havinga relatively high melting-point, such as titanium and molybdenum.

Through the aforementioned processes, the capacitor 148 is fabricated.

2. Transistor

Next, the second insulating film 196 is formed so as to cover the gateelectrode 168 and the second electrode 172 (FIG. 6C). The secondinsulating film 196 may have either a single-layer structure or astacked-layer structure.

The second insulating film 196 serves as the gate insulating film of thefirst transistor 140. Therefore, it is preferred to use a siliconoxide-containing insulating film as the second insulating film 196 inorder to suppress carrier generation in the semiconductor film 180formed over the second insulating film 196. When the second insulatingfilm 196 has a stacked structure, a region in contact with thesemiconductor film 180 preferably contains silicon oxide.

The second insulating film 196 may be formed with the same method asthat of the first insulating film 194 and include the same material asthat of the first insulating film 194. When the second insulating film196 is formed, it is preferred that an atmosphere contain as littlehydrogen-containing gas such as hydrogen gas and water vapor aspossible, by which the second insulating film 196 with a small hydrogencomposition and an oxygen composition close to or larger thanstoichiometry can be formed.

After forming the second insulating film 196, an ion-implantationtreatment or an ion-doping treatment is performed on the semiconductorfilm 160 by using the gate electrode 168 as a mask. An element impartinga p-type conductivity, such as boron and aluminum, or an elementimparting an n-type conductivity, such as phosphorus and nitrogen, isrepresented as an ion. This process allows the formation of a channelregion in a region overlapping with the gate electrode 168 and asource/drain region in another region of the semiconductor film 160.Note that an ion-implantation treatment or an ion-doping treatment maybe carried out on the third transistor 144 and the fourth transistor 146as appropriate.

Next, the semiconductor film 180 is formed over the second insulatingfilm 196 so as to overlap with the second electrode 172 (FIG. 7A). Thesemiconductor film 180 may include an oxide semiconductor which can beselected from Group 13 elements such as indium and gallium. Thesemiconductor film 180 may include a plurality of different Group 13elements and may be IGO. The semiconductor film 180 may further containGroup 12 elements and is exemplified by IGZO. The semiconductor film 180may include another element such as tin of Group 14 elements andtitanium and zirconium of Group 4 elements.

The semiconductor film 180 is formed by utilizing a sputtering methodand the like at a thickness of 20 nm to 80 nm or 30 nm to 50 nm. When asputtering method is applied, the film formation can be conducted underan atmosphere containing oxygen gas, such as a mixed atmosphere of argonand oxygen gas. In this case, a partial pressure of argon may be lowerthan that of oxygen gas.

The semiconductor film 180 preferably possesses few crystal defects suchas an oxygen defect. Hence, it is preferred to perform a heat treatment(annealing) on the semiconductor film 180. The heat treatment may beconducted before patterning or after patterning the semiconductor film180. It is preferred that the heat treatment be performed beforepatterning because the oxide semiconductor film 180 may decrease involume (shrinking) by the heat treatment. The heat treatment may beconducted in the presence of nitrogen, dry air, or atmospheric air at anormal pressure or a reduced pressure. The heating temperature can beselected from a range of 250° C. to 500° C. or 350° C. to 450° C., andthe heating time can be selected from a range of 15 minutes to 1 hour.However, the heat treatment can be conducted outside these temperatureand time ranges. Oxygen is introduced or migrated to the oxygen defectsof the semiconductor film 180 by the heat treatment, which results inthe semiconductor film 180 having a well-defined structure, a smallnumber of crystal defects, and high crystallinity. Accordingly, thefirst transistor 140 having high reliability and excellent electricalproperties such as a low off current and low property (thresholdvoltage) variation.

Next, as shown in FIG. 7B, the first insulating film 194 and the secondinsulating film 196 are processed with etching to form opening portionsexposing the semiconductor film 160 and the first electrode 170. Afterthat, the image-signal line 122, the second drain electrode 166, thefirst source electrode 182, and the first drain electrode 184 are formedto fill the opening portions (FIG. 7C). These wirings and electrodes canbe formed by applying a similar material, structure, and method to thosefor the formation of the gate electrode 168 and the first electrode 170and the second electrode 172 of the capacitor 148 and can exist in thesame layer as one another. The image-signal line 122 also functions asthe source electrode (second source electrode) of the second transistor142. Note that a thickness of the semiconductor film 180 in the channelregion may be smaller than that in a region covered by the first sourceelectrode 182 or the first drain electrode 184. Although not shown, aninsulting film for protecting the channel may be provided between thesemiconductor film 180 and the first source electrode 182 and the firstdrain electrode 184.

With the above processes, the first transistor 140 and the secondtransistor 142 are fabricated as well as the third transistor 144 andthe fourth transistor 146.

3. Display Element

Next, the leveling film 198 is formed so as to cover the firsttransistor 140 and the second transistor 142 (FIG. 8A). The levelingfilm 198 can be formed by using an organic insulator. A polymer materialsuch as an epoxy resin, an acrylic resin, a polyimide, a polyamide, apolyester, a polycarbonate, and a polysiloxane is represented as anorganic insulator, and the leveling film 198 can be formed with awet-type film-forming method such as a spin-coating method, an ink-jetmethod, a printing method, and a dip-coating method. The leveling film198 may have a stacked structure including a layer containing theaforementioned organic insulator and a layer containing an inorganicinsulator. In this case, a silicon-containing inorganic insulator suchas silicon oxide, silicon nitride, silicon nitride oxide, and siliconoxynitride is represented as an inorganic insulator, and the layercontaining an inorganic insulator can be formed with a sputtering methodor a CVD method.

Next, the leveling film 198 is processed to form an opening portionexposing the first drain electrode 184 (FIG. 8B). After that, the pixelelectrode 152 of the display element 150 is formed so as to cover theopening portion and be electrically connected to the first drainelectrode 184.

When light emission from the display element 150 is extracted throughthe substrate 102, a material having a light-transmitting property, suchas a conductive oxide exemplified by ITO and IZO, can be used for thepixel electrode 152. On the other hand, when the light emission from thedisplay element 150 is extracted from a side opposite to the substrate102, a metal such as aluminum and silver or an alloy thereof can beused. Alternatively, a stacked layer of the aforementioned metal oralloy and the conductive oxide can be employed. For example, a stackedstructure in which a metal is sandwiched by a conductive oxide (e.g.,ITO/silver/ITO etc.) can be used.

Next a partition wall 200 is formed (FIG. 9A). The partition wall 200 isformed so as to cover an edge portion of the pixel electrode 152 and theopening portion formed in the leveling film 198 and has a function toabsorb steps caused by the edge portion and the opening portion and toelectrically insulate the pixel electrodes 152 of the adjacent pixels104 from each other. The partition wall 200 is also called a bank (rib).The partition wall 200 can be formed by using a material usable in theleveling film 198, such as an epoxy resin and an acrylic resin. Thepartition wall 200 has an opening portion so as to expose a part of thepixel electrode 152. An edge portion of the opening preferably has amoderately tapered shape because a steep slope in the edge portion ofthe opening portion may cause a defect in the EL layer 154 and theopposing electrode 156 formed later.

Next, the EL layer 154 is formed over the pixel electrode 152 (FIG. 9B).The EL layer 154 is formed so as to be in contact with the exposedportion of the pixel electrode 152 and cover at least a part of thepartition wall 200. In the present specification and claims, an EL layermeans all the layers sandwiched between a pair of electrodes and may bestructured by a single layer or a plurality of layers. For example, theEL layer 154 can be structured by appropriately combining acarrier-injection layer, a carrier-transporting layer, an emissionlayer, a carrier-blocking layer, an exciton-blocking layer, and thelike. Moreover, the EL layer 154 may be different in structure betweenadjacent pixels 104. For example, the EL layer 154 may be formed so thatthe emission layer is different but other layers are the same instructure between the adjacent pixels 104. With this structure,different emission colors can be obtained from the adjacent pixels 104,and full color display can be realized. On the contrary, the same ELlayer 154 may be used in all pixels 104. In this case, the EL layer 154giving white emission may be formed so as to be shared by all pixels104, and the wavelength of the light extracted from each pixel 104 maybe selected by using a color filter and the like. The EL layer 154 canbe formed by applying an evaporation method or the aforementionedwet-type film-formation method.

Next, the opposing electrode 156 is formed over the EL layer 154 (FIG.9B). The display element 150 is structured by the pixel electrode 152,the EL layer 154, and the opposing electrode 156. Carriers (electronsand holes) are injected to the EL layer 154 from the pixel electrode 152and the opposing electrode 156, and the light-emission is obtainedthrough a process in which an excited state generated by therecombination of the carriers relaxes to a ground state. Therefore, inthe display element 150, a region in which the EL layer 154 and thepixel electrode 152 are in direct contact with each other is an emissionregion.

When the light emission from the display element 150 is extractedthrough the substrate 102, a metal such as aluminum and silver or analloy thereof can be used for the opposing electrode 156. On the otherhand, when the light-emission from the display element 150 is extractedthrough the opposing electrode 156, the opposing electrode 156 is formedby using the aforementioned metal or alloy so as to have a thicknesswhich allows visible light to pass therethrough. Alternatively, amaterial having a light-transmitting property, such as a conductiveoxide exemplified by ITO, IZO, and the like, can be used for theopposing electrode 156. Furthermore, a stacked structure of theaforementioned metal or alloy with the conductive oxide (e.g.,MG-Ag/ITO, etc.) can be employed in the opposing electrode 156. Theopposing electrode 156 can be formed with an evaporation method, asputtering method, and the like.

With the above processes, the display element 150 is fabricated.

A passivation film (sealing film) 206 may be disposed over the opposingelectrode 156 as an optional structure (FIG. 10). One of the functionsof the passivation film 206 is to prevent water from entering theprecedently prepared display element 150 from outside, and thepassivation film 206 is preferred to have a high gas-barrier property.For example, it is preferred that the passivation film 206 be formed byusing an inorganic material such as silicon nitride, silicon oxide,silicon nitride oxide, and silicon oxynitride. Alternatively, an organicresin including an acrylic resin, a polysiloxane, a polyimide, apolyester, and the like may be used. In the structure illustrativelyshown in FIG. 10, the passivation film 206 has a three-layer structureincluding a first layer 208, a second layer 210, and a third layer 212.

Specifically, the first layer 208 may include an inorganic insulatorsuch as silicon oxide, silicon nitride, silicon nitride oxide, andsilicon oxynitride and may be formed by applying a CVD method or asputtering method. As a material for the second layer 210, a polymermaterial selected from an epoxy resin, an acrylic resin, a polyimide, apolyester, a polycarbonate, a polysiloxane, and the like can be used.The second layer 210 can be formed with the aforementioned wetfilm-forming method. Alternatively, the second layer 210 may be formedby atomizing or gasifying oligomers functioning as a raw material of thepolymer material at a reduced pressure, spraying the first layer 208with the oligomers, and polymerizing the oligomers. At this time, apolymerization initiator may be mixed in the oligomers. Additionally,the first layer 208 may be sprayed with the oligomers while cooling thesubstrate 102. The third layer 212 can be formed by applying the samematerial and method as those for the first layer 208.

Although not illustrated, the opposing substrate 116 may be arrangedover the passivation film 206 as an optional structure (see, FIG. 1).The opposing substrate 116 is fixed to the substrate 102 with anadhesive. In this case, a space between the opposing substrate 116 andthe passivation film 206 may be filled with an inert gas or a fillersuch as a resin. Alternatively, the passivation film 206 and theopposing substrate 116 may be directly adhered with an adhesive. When afiller is used, the filler is preferred to have a high transmittingproperty with respect to visible light. When the opposing substrate 116is fixed to the substrate 102, a gap therebetween may be adjusted byadding a spacer in the adhesive or the filler. Alternatively, astructure functioning as a spacer may be formed between the pixels 104.

Furthermore, a light-shielding film having an opening in a regionoverlapping with the emission region and a color filter in a regionoverlapping with the emission region may be disposed over the opposingsubstrate 116. The light-shielding film is formed by using a metal witha relatively low reflectance, such as chromium and molybdenum, or amixture of a resin material with a coloring material having a black orsimilar color. The light-shielding film has a function to shield thescattered or reflected external light and the like other than the lightdirectly obtained from the emission region. The color filter can beformed while changing its optical properties between the adjacent pixels104 so that red emission, green emission, and blue emission areextracted. The light-shielding film and the color filter may be providedover the opposing substrate 116 with an undercoat film interposedtherebetween, and an overcoat layer may be further arranged to cover thelight-shielding film and the color filter.

Through the above processes, the display device 100 described in theFirst Embodiment is manufactured.

Third Embodiment

In the present embodiment, a display device 300 according to anembodiment of the present invention is explained by using FIG. 11.Explanation of the contents duplicated in the First and SecondEmbodiments may be omitted.

A schematic cross-sectional view of the display device 300 is shown inFIG. 11. FIG. 11 corresponds to the cross-sections along chain lines A-Band C-D in FIG. 4. A difference from display device 100 is that thedisplay device 300 possesses, over the first transistor 140, the firstsource electrode 182, the first drain electrode 184, and a thirdinsulating film 186 in contact with the semiconductor film 180. Thethird insulating film 186 extends to the second transistor 142 and tothe third transistor 144 and the fourth transistor 146 which are notillustrated and is sandwiched between the image-signal line 122 servingas the second source electrode and the second insulating film 196 andbetween the second drain electrode 166 and the second insulating film196.

The third insulating film 186 may contain the same material as that ofthe second insulating film 196 and can be formed by applying the sameformation method as that for the second insulating film 196. Similar tothe second insulating film 196, it is also preferred that the thirdinsulating film 186 contain silicon oxide in order to suppress carriergeneration in the semiconductor film 180.

In the present embodiment, the first source electrode 182 and the firstdrain electrode 184 are formed over the semiconductor film 180, and thenthe third insulating film 186 is formed before forming the second sourceelectrode (image-signal line 122) and the second drain electrode 166.Sequentially, the first insulating film 194, the second insulating film196, and the third insulating film 186 are simultaneously subjected toetching processing to form opening portions exposing the semiconductorfilm 160, and the second source electrode (image-signal line 122) andthe second drain electrode 166 are formed in the opening portions.

The use of such a structure enables prevention of loss and contaminationof the semiconductor film 180 when the opening portions are formed inthe first insulating film 194, the second insulating film 196, and thethird insulating film 186. Furthermore, in the case where an oxide film,which is formed on a surface of the semiconductor film 160 after beingexposed, is removed with a strong acid such as hydrofluoric acid, thesemiconductor film 180 is prevented from being lost or contaminated.

Similar to the display device 100, the display device 300 possesses thefirst transistor 140 which is stacked over the capacitor 148 and has thesemiconductor film 180 containing an oxide semiconductor. Therefore,high-quality images can be realized because of the small variation incharacteristics of a transistor having an oxide semiconductor.Additionally, the stack of the capacitor 148 with the first transistor140 results in a large channel width, which allows a large current toflow in the display element 150, providing an image at a high luminance.Furthermore, the capacitor 148 having a large capacitance can befabricated, by which a writing frequency of the image signal Vsig can bereduced. Accordingly, power consumption can be decreased.

Fourth Embodiment

In the present embodiment, a display device 400 according to anembodiment of the present invention is explained by using FIG. 1, FIG.2, FIG. 12, and FIG. 13. Explanation of the contents duplicated in theFirst to Third Embodiments may be omitted.

Similar to the display device 100, the display device 400 also has aplurality of pixels 104 (FIG. 1 and FIG. 2). However, as demonstrated byan equivalent circuit of FIG. 12, the pixel 104 of the display device400 is different from that of the display device 100 in that the drainelectrode of the fourth transistor 146 used to initialize the firsttransistor 140 is connected to the drain electrode of the thirdtransistor 144 and the source electrode of the first transistor 140.Hence, it is not necessary to provide the fourth transistor 146 in eachpixel 104, and the fourth transistor 146 can be disposed in the drivercircuits 108. Accordingly, it is possible to decrease the size of eachpixel 104 and improve an aperture ratio thereof.

As shown in FIG. 13, the reset power-source line 130 extendssubstantially parallel to the first scanning line 120 and the secondscanning line 126 in the display device 400 and is connected to thefourth transistor 146 disposed in the driver circuits 108 shown inFIG. 1. Hence, the reset power-source line 130 can exist in the samelayer as the first scanning line 120 and the second scanning line 126.The reset power-source line 130 is connected to the first sourceelectrode 182 of the first transistor 140 through a contact hole.

The first transistor 140 is arranged over the capacitor 148 of thedisplay device 400 and is able to possess a large channel widthsimilarly to the display device 100. Therefore, a large current can beflowed in the first transistor 140 even if an oxide semiconductor isused in the semiconductor film 180, thereby enabling the display element150 to be driven at a high luminance. Additionally, the use of an oxidesemiconductor in the semiconductor film 180 reduces variation incharacteristics between the pixels 104. Hence, the present embodimentallows a high-quality image to be displayed at a high luminance.

Fifth Embodiment

In the present embodiment, a display device 500 according to anembodiment of the present invention is explained by using FIG. 1, FIG.2, FIG. 14, and FIG. 15. Explanation of the contents duplicated in theFirst to

Fourth Embodiments may be omitted.

Similar to the display device 100, the display device 500 also has aplurality of pixels 104 (FIG. 1 and FIG. 2). As shown in FIG. 14, unlikethe display device 100, the display device 500 further possesses a thirdelectrode 188 overlapping with the semiconductor film 180. The thirdelectrode 188 may be connected to the second electrode 172 through acontact hole. In this case, the same potential is applied to the secondelectrode 172 and the third electrode 188. Thus, the first transistor140 has a structure having two gate electrodes over and under thesemiconductor film 180.

FIG. 15 shows a schematic drawing of a cross-section along a chain lineG-H of FIG. 14. The third insulating film 186 is provided over thesemiconductor film 180, and the third electrode 188 functioning as agate electrode is formed thereover. The display device 500 furtherpossesses a fourth insulating film 202 over the third electrode 188, andthe first source electrode 182 and the first drain electrode 184 areelectrically connected to the semiconductor film 180 through contactholes formed in the third insulating film 186 and the fourth insulatingfilm 202.

The third insulating film 186 and the fourth insulating film 202 can beformed in a similar structure with a similar method to those of thethird insulating film 186 of the Third Embodiment, and the thirdelectrode 188 can be formed in a similar structure with a similar methodto those of the second electrode 172.

In the display device 500 of the present embodiment, since channels areformed in an upper portion and a lower portion of the semiconductor film180, the channel width is expanded in appearance and a larger currentcan be flowed. As a result, a high-quality image can be supplied at ahigh luminance. Additionally, provision of two gate electrodes allowsthe threshold shift to be controlled more effectively, enabling theproduction of a highly reliable display device.

Sixth Embodiment

In the present embodiment, a display device 600 according to anembodiment of the present invention is explained by using FIG. 1 to FIG.3,

FIG. 16, and FIG. 17. Explanation of the contents duplicated in theFirst to Fifth Embodiments may be omitted.

Similar to the display device 100, the display device 600 also has aplurality of pixels 104 (FIG. 1 and FIG. 2), and the equivalent circuitof the pixel 104 is also the same as that shown in FIG. 3. Differencesfrom the First Embodiment are that, as shown in FIG. 16, the firstscanning line 120 is located between the second scanning line 126 andthe third scanning line 128 and that a direction of a current flowing inthe first transistor 140 is perpendicular to a direction in which thefirst scanning line 120, the second scanning line 126, and the thirdscanning liner 128 extend.

A schematic drawing of a cross-section along a chain line J-K of FIG. 16is shown in FIG. 17. In the display device 600, the first transistor 140is placed over the capacitor 148, and the second electrode 172 is sharedby the capacitor 148 and the first transistor 140. The first electrode170 and the second electrode 172 of the capacitor 148 and thesemiconductor film 180 overlap with one another.

In the display device 600, similar to the display device 100, thecapacitor 148 and the first transistor 140 are stacked together. The useof an oxide semiconductor in the semiconductor film 180 of the firsttransistor 140 allows a high-quality image to be realized due to thesmall variation in characteristics of a transistor having an oxidesemiconductor. Additionally, the stack of the capacitor 148 with thefirst transistor 140 results in a large channel width, which enables alarge current to flow in the first transistor 140 even if an oxidesemiconductor is employed in the semiconductor film 180. As a result, itis possible to flow a large current in the display element 150, enablingan image to be provided at a high luminance. Furthermore, the capacitor148 having a large capacitance can be fabricated, by which a writingfrequency of the image signal Vsig can be reduced. Accordingly, powerconsumption can be decreased.

The aforementioned modes described as the embodiments of the presentinvention can be implemented by appropriately combining with each otheras long as no contradiction is caused. Furthermore, any mode which isrealized by the persons ordinarily skilled in the art through theappropriate addition, deletion, or design change of elements or throughthe addition, deletion, or condition change of a process is included inthe scope of the present invention as long as they possess the conceptof the present invention.

In the specification, although the cases of the organic EL displaydevice are exemplified, the embodiments can be applied to any kind ofdisplay devices of the flat panel type such as other self-emission typedisplay devices, liquid crystal display devices, and electronic papertype display device having electrophoretic elements and the like. Inaddition, it is apparent that the size of the display device is notlimited, and the embodiment can be applied to display devices having anysize from medium to large.

It is properly understood that another effect different from thatprovided by the modes of the aforementioned embodiments is achieved bythe present invention if the effect is obvious from the description inthe specification or readily conceived by the persons ordinarily skilledin the art.

What is claimed is:
 1. A display device comprising: a capacitorcomprising: a first electrode; a first insulating film over the firstelectrode; and a second electrode over the first insulating film; and afirst transistor over the capacitor, the first transistor comprising:the second electrode; a second insulating film over the secondelectrode; an oxide semiconductor film over the second insulating film;and a first source electrode and a first drain electrode over the oxidesemiconductor film, the first source electrode and the first drainelectrode being electrically connected to the oxide semiconductor film.2. The display device according to claim 1, wherein the second electrodeis shared by the capacitor and the first transistor.
 3. The displaydevice according to claim 1, wherein the first electrode includessilicon.
 4. The display device according to claim 1, wherein the oxidesemiconductor film overlaps with the second electrode.
 5. The displaydevice according to claim 1, wherein an area of the oxide semiconductorfilm is smaller than an area of the second electrode.
 6. The displaydevice according to claim 1, further comprising a second transistor, thesecond transistor comprising: a semiconductor film under the firstinsulating film; a gate electrode over the first insulating film; thesecond insulating film over the gate electrode; and a second sourceelectrode and a second drain electrode over the second insulating film,the second source electrode and the second drain electrode beingelectrically connected to the semiconductor film, wherein thesemiconductor film comprises silicon.
 7. The display device according toclaim 6, wherein one of the second source electrode and the second drainelectrode is electrically connected to the second electrode.
 8. Thedisplay device according to claim 6, wherein the first source electrode,the first drain electrode, the second source electrode, and the seconddrain electrode exist in the same layer.
 9. The display device accordingto claim 6, further comprising an interlayer film over the first sourceelectrode and the first drain electrode, wherein the interlayer film issandwiched between the second source electrode and the second insulatingfilm and between the second drain electrode and the second insulatingfilm.
 10. The display device according to claim 6, further comprising alight-emitting element electrically connected to one of the first sourceelectrode and the first drain electrode.
 11. A display devicecomprising: a capacitor comprising: a first electrode; a firstinsulating film over the first electrode; and a second electrode overthe first insulating film; and a first transistor over the capacitor,the first transistor comprising: the second electrode; a secondinsulating film over the second electrode; an oxide semiconductor filmover the second insulating film; a third insulating film over the oxidesemiconductor film; a third electrode over the third insulating film;and a first source electrode and a first drain electrode over the thirdelectrode, the first source electrode and the first drain electrodebeing electrically connected to the oxide semiconductor film.
 12. Thedisplay device according to claim 11, wherein the third electrodeoverlaps with the oxide semiconductor film.
 13. The display deviceaccording to claim 11, wherein the second electrode and the thirdelectrode are electrically connected to each other.
 14. The displaydevice according to claim 11, wherein the second electrode is shared bythe capacitor and the first transistor.
 15. The display device accordingto claim 11, wherein the oxide semiconductor film overlaps with thesecond electrode.
 16. The display device according to claim 11, whereinan area of the oxide semiconductor film is smaller than an area of thesecond electrode.
 17. The display device according to claim 11, furthercomprising a second transistor, the second transistor comprising: asemiconductor film under the first insulating film; a gate electrodeover the first insulating film; the second insulating film over the gateelectrode; and a second source electrode and a second drain electrodeover the second insulating film, the second source electrode and thesecond drain electrode being electrically connected to the semiconductorfilm, wherein the semiconductor film comprises silicon.
 18. The displaydevice according to claim 17, wherein one of the second source electrodeand the second drain electrode is electrically connected to the secondelectrode.
 19. The display device according to claim 17, wherein thefirst source electrode, the first drain electrode, the second sourceelectrode, and the second drain electrode exist in the same layer. 20.The display device according to claim 17, further comprising aninterlayer film over the first source electrode and the first drainelectrode, wherein the interlayer film is sandwiched between the secondsource electrode and the second insulating film and between the seconddrain electrode and the second insulating film.